(1) Field of the Invention
The present invention relates to a heating device suitably used for the fixing unit of a dry type electrophotographic apparatus, the drying device of a wet type electrophotographic apparatus, the drying device of an ink-jet printer, a rewritable media erasing device and the like, as well as relating to an image forming apparatus using this heating device.
(2) Description of the Prior Art
In a typical heating device for the electrophotographic process, the toner on the paper surface as a recording medium is fixed on the paper by fusing and solidification. While paper of a smaller width compared to the heating width of the heating element is passed through the heating device, heat energy is supplied from the heat source also to the non-paper feed areas through which paper of a smaller width does not pass. However, since there is no paper to which heat can transfer in the non-paper feed areas, heat builds up in these areas, causing excessive temperature rise therein.
If paper of a greater width than the small-width paper is passed through under this condition, deficiencies as follows will occur due to the temperature imbalance between the paper feed area (at the middle) of the small-width paper and the non-paper feed areas (at both sides).
First, due to temperature imbalance, when paper of greater width is passed through, it becomes wrinkled at the areas corresponding to the non-paper feed areas of small-width paper.
Second, since the non-paper feed areas are overheated, if paper with an unfixed image thereon is passed through, to be fixed, under this condition, image deficiencies such as high-temperature offset will occur.
Third, since the heat roller surface is coated with an non-stick layer of fluororesin or the like in order to improve the separability when fixing, this non-stick layer will be heated to high temperatures in the overheated areas, causing heat deterioration and lowering of its durability.
Generally, in order to assure adhesion between the non-stick layer and the metal core of the heat roller, a primer such as a silicon resin adhesive, undercoating or the like is provided. Therefore, due to degradation of the primer itself and due to degradation of the non-stick layer, the bonding strength between the non-stick layer and the primer or that between the primer and the metal core lowers, as a result the non-stick layer and/or primer may peel off.
To solve the above problems, there have been several conventional methods proposed: examples include a method of lowering the surface temperature by forcibly rotating the heating element after passage of printing paper (e.g., Japanese Patent Application Laid-Open Hei 8 No. 21779), a method of naturally cooling the heating element surface by holding the heating element still after passage of printing paper (e.g., Japanese Patent 2696908) and a method of forcibly cooling the overheated parts or the whole of the heating element using a blower such as a fan etc. Further, in order not to generate inefficient heat in the non-paper feed areas during printing, there has been another approach (Japanese Patent Application Laid-Open Sho 60 No. 22164), with which the heat roller is adapted to incorporate divided, or multiple parts, of heat sources in conformity with the sizes of paper that pass over the heat sources.
Next, these prior art techniques will be described in further detail.
(1) The Method Disclosed in Japanese Patent Application Laid-Open 8 No. 211779:
The method disclosed in Japanese Patent Application Laid-Open 8 No. 211779 is aimed at providing a compact and economic fixing unit in which improvements against fixing defects, offsets and the like attributed to the temperature distribution imbalance across the heat roller are made.
More specifically, the controller for regulating the temperature distribution across the heat roller of the fixing unit, as it is commanded to start a new job, estimates the current temperature distribution across the heat roller based on the information as to the previous job and the elapsed time from the end of the job and judges whether the new job is permitted under the present conditions.
If the controller has determined that the heat roller is not uniform in temperature, the controller performs its control such that the heat roller is idly rotated for a fixed period of time before start of the next job, the set temperature is changed before start of the next job, or all the operations are prohibited for a fixed period.
By effecting such control, the next copy job can be started after the heat roller has become uniform in the temperature distribution, without being affected by the previous copying job.
In the above cooling method, there is a risk that the heating element might be partially reduced in temperature to a temperature lower than the functional fixing temperature range because the heating element which has been uneven in temperature is cooled. To lessen this possibility, there is also a proposed method in which the heating element as a whole is heated after the cooling.
(2) The Method Disclosed in Japanese Patent 2696908:
In the method disclosed in Japanese Patent 2696908, if paper of a smaller size than B5, e.g., postcards or smaller, is detected, the copying operation of the small-sized paper alone is halted for a predetermined period, whereby the non-paper feed areas in the heat roller of the fixing unit are inhibited from being elevated in temperature.
More explicitly, in an image forming apparatus having a roller type fixing unit made up of a heat roller and pressing roller put in pressing contact with each other, continuous copying operations of paper of a size smaller than B5 such as postcards, are allowed until the temperature of the non-paper feed areas on the heat roller of the fixing unit becomes elevated to a predetermined temperature, and then after reaching the predetermined temperature, copying operation of the small-size paper alone is halted over a predetermined period of time so that the non-paper feed areas of the heat roller will not become overheated. This halt is continued until the temperature distribution across the heat roller becomes uniform.
(3) The Method Disclosed in Japanese Patent Application Laid-Open Sho 60 No. 22164:
In the method disclosed in Japanese Patent Application Laid-Open Sho 60 No. 22164, temperature control in conformity with the recording paper size is enabled in order not to generate unnecessary heat in the non-paper feed areas during printing.
More clearly, in conformity with the size of recording paper conveyed through the fixing unit, multiple heater elements arranged in the fixing unit are selectively energized. Further, the power of each heater element is also controlled so as to optimize the temperature distribution.
As stated above, in the method disclosed in Japanese Patent Application Laid-Open Hei 8 No. 211779, when the surface temperature of the heat roller has risen, to decrease the temperature the paired rollers, i.e., heat roller and pressing roller, are idly turned for forcible cooling.
However, in order to decrease the temperature difference between the paper feed area and the non-paper feed areas to a small enough level, it is necessary to idly rotate the heat roller for a long time. Accordingly, extra time is needed until the next copying operation is allowed to start, resulting in reduction in throughput.
In the method disclosed in Japanese Patent 2696908, upon a copying operation using small-sized recording paper, the copying operation of small-sized paper alone is halted for a predetermined period, so that the temperature of the heat roller can fall within the range in which copying operations can be implemented.
Solitary prohibition of the copying operation of small sized paper for the predetermined period of time makes it possible to reduce the temperature difference between the paper feed area and the non-paper feed areas to a certain small level. However, it is necessary to take a long halt in order to reduce the temperature of the non-paper feed areas to a level at which the copying operation is allowed. Accordingly, extra time is needed until the next copying operation is allowed to start. That is, this configuration needs long inactive time hence long intervals between copying operations, resulting in reduction in throughput.
In the method disclosed in Japanese Patent Application Laid-Open Sho 60 No. 22164, multiple heat sources are used in conformity with the paper size so as to prevent temperature rise in the non-paper feed areas. Nevertheless, since this configuration does not have any efficient cooling means in combination, reduction in temperature cannot be achieved fast enough, hence it is impossible to obtain satisfactory effect in spite of increase in cost due to provision of multiple heat sources.
The present invention has been devised in view of the above problems and it is therefore an object of the present invention to provide a heating device, as well as an image forming apparatus using it, which can quickly restore a heat roller from an overheated state, without causing any paper wrinkles or causing any image degradation and can make control so as to uniformly keep the overall temperature distribution across the heat roller within a predetermined temperature range.
In order to achieve the above object the heating device according to the present invention and the image forming apparatus using it are configured as follows:
In accordance with the first aspect of the present invention, a heating device having a heating element including a heat source and a pressing element put in pressing contact with the heating element, wherein recording media are passed through and between the two elements so as to heat the media, the heating device comprises: a rotational drive means for rotating the heating element and pressing element; and a control means for making control of each part so as to implement a cooling process for cooling the heating element, and is characterized in that when the final recording medium in a consecutive heating operation of recording media of a solitary size has passed through and between the heating element and pressing element, the control means implements two different modes in combination in accordance with the size of the recording media, the rotational mode in which the heating element and pressing element are rotated by the rotational drive means for a predetermined period of time and the stationary mode in which the heating element and pressing element are stopped rotating by the rotational drive means for a predetermined period of time.
In accordance with the second aspect of the present invention, the heating device having the above first feature is characterized in that the control means implements the stationary mode after the operation in the rotational mode.
In accordance with the third aspect of the present invention, the heating device having the above first feature is characterized in that the control means implements the rotational mode after the operation in the stationary mode.
In accordance with the fourth aspect of the present invention, the heating device having the above first feature is characterized in that the control means deactivates the heat source while the operation is being implemented in at least one of the modes, the rotational and stationary modes.
In accordance with the fifth aspect of the present invention, the heating device having the above first feature is characterized in that the control means makes control during the cooling process so that the temperature of the heating element is maintained so as to fall within a predetermined range.
In accordance with the sixth aspect of the present invention, the heating device having the above fifth feature is characterized in that the control means set the operational conditions for the cooling process, based on the optimal cooling process conditions stored beforehand and the recording media information at least including the size of recording media and the number of media in the previous heating process.
In accordance with the seventh aspect of the present invention, the heating device having the above fifth feature is characterized in that the control means makes control so as to keep the temperature within the predetermined range when the operation is implemented in the stationary mode.
In accordance with the eighth aspect of the present invention, the heating device having the above first feature further includes: a recording media size detecting means for detecting the size of recording media and is characterized in that when the control means, after a previous heat process has been finished, confirms that a subsequent heat process should be implemented, the control means implements the cooling process if the recording media size detecting means indicates that the media size of the subsequent heat process is greater than that of the previous heat process, and the control means will not implement the cooling process if the media size of the subsequent heat process is equal to or smaller than that of the previous heat process.
In accordance with the ninth aspect of the present invention, the heating device having the above fifth feature is characterized in that the control means, after completion of the cooling process, actuates an energy save mode operation in which the temperature range of the heating element is shifted to another temperature range which is slightly lower to a certain degree than the predetermined temperature range and can be immediately restored to the predetermined temperature range.
In accordance with the tenth aspect of the present invention, the heating device having the above first feature is characterized in that the control means makes control such that the cooling process is stopped in accordance with the size of recording media passing through and between the heating element and the pressing element.
In accordance with the eleventh aspect of the present invention, the heating device having the above first feature is characterized in that the control means makes control so that the rotational mode and stationary mode are repeated alternately a multiple number of times.
In accordance with the twelfth aspect of the present invention, the heating device having the above first feature is characterized in that when the control means determines that the temperature of the heating element has been elevated, deviating from the predetermined temperature range, the control means makes control so that the rotational mode starts first.
In accordance with the thirteenth aspect of the present invention, the heating device having the above first feature is characterized in that when the control means determines that the mean temperature of the heating element falls within the predetermined range but the spatial temperature distribution has strong fluctuations, the control means makes control so that the stationary mode starts first.
In accordance with the fourteenth aspect of the present invention, the heating device having the above fifth feature is characterized in that the heating element includes a multiple number of heat sources assigned for different heating areas, and the control means makes temperature control of each heat source corresponding to an individual heating area, independently from others.
In accordance with the fifteenth aspect of the present invention, the heating device having the above first feature further includes a temperature detecting means for measuring the temperature of the heating element and is characterized in that the control means sets the operational conditions for the cooling process, based on the temperature information obtained from the temperature detecting means.
In accordance with the sixteenth aspect of the present invention, the heating device having the above fifth feature further includes a temperature detecting means for measuring the temperature of the heating element and is characterized in that the control means sets the operational conditions for the cooling process, based on the temperature information obtained from the temperature detecting means.
In accordance with the seventeenth aspect of the present invention, an image forming apparatus for forming toner images on recording media, includes, as a fixing unit for fixing toner images on the recording media, a heating device comprising: a heating element including a heat source; a pressing element put in pressing contact with the heating element; a rotational drive means for rotating the heating element and pressing element so as to pass the recording media through and between the two elements so as to heat the media; and a control means for making control of each part so as to implement a cooling process for cooling the heating element, wherein when the final recording medium in a consecutive heating operation of recording media of a solitary size has passed through and between the heating element and pressing element, the control means implements two different modes in combination in accordance with the size of the recording media, the rotational mode in which the heating element and pressing element are rotated by the rotational drive means for a predetermined period of time and the stationary mode in which the heating element and pressing element are stopped rotating by the rotational drive means for a predetermined period of time.
In the heating device according to the present invention, when the final recording medium has passed through the heating device, a cooling and post process is effected by combination of the rotational mode in which the heating element and pressing element are rotated for a predetermined period of time and the stationary mode in which the heating element and pressing element are stopped rotating for a predetermined period of time. In this way, the cooling process is effected by implementing the two modes in combination, hence it is possible to lower the surface temperature of the heating element and pressing element, more quickly compared to the prior art techniques.
Further, the two modes produce individual influences different from each other when the surface temperatures of the heat and pressing rollers are lowered. Specifically, the rotational mode functions such that the differential temperature between the non-media feed areas which are overheated and the media feed area cannot be reduced to a small enough level but the maximum temperature in the non-media feed areas lowers or the temperature across the whole part totally lowers at a high temperature drop rate. On the other hand, the stationary mode functions such that the differential temperature between the non-media feed areas and the media feed area can be markedly reduced compared to the rotational mode.
Accordingly, it is possible to lower the temperature in the non-media feed areas which is overheated more quickly by implementing the rotational mode and the stationary mode for predetermined periods specified in accordance with the size of recording media. Therefore, it is possible to quickly restore the normal state from the condition in which occurrence of wrinkles and image deficiencies such as high-temperature offset may arise as well as avoiding reduction in throughput of image forming.